1. Field of the Invention
This invention relates to an apparatus for improving color correctness of an LCD, and particularly to a gamma correction apparatus for adjusting gamma curve separation to improve color correctness.
2. Description of Related Art
Along with enormous promotions of thin film transistor (TFT) fabrication technique, liquid crystal displays (LCD) are broadly applied to personal digital assistants (PDA), notebooks (NB), digital cameras (DC), digital videos (DV), mobile phones, etc. In order to form images, a liquid crystal (LC) driving circuit is usually used in an LCD to decode input signals so as to form displaying data and scanning data, and further to control the displaying of the LCD.
FIG. 1 depicts a relationship of transparency of typical red (“R”), green (“G”), and blue (“B”) pixel devices with respect to the applied electric field. Three separate data curves in FIG. 1 imply that the LC layer in the pixel devices presents various refractivity and retardation values with respect to the passing-through visible light beam under some identical applied electric fields. That is, the RGB pixel devices show different transparency.
Generally, in order to provide a quality look-up feeling to human eyes, a gamma curve of FIG. 2, compared with the graph of FIG. 1, shows a relationship between transparency and bit numbers of the pixel devices, wherein the bit number represents the brightness feeling of human eyes. As shown in FIG. 2, the RGB gamma curves are separated to each other, by which it would be difficult to keep the combination of RGB illumination at a preset white-balance point. Also, bias in image will definitely exist in response to the inputted displaying signals.
In the art to solve the separation problem in gamma curves as shown in FIG. 1, a typical method to have the R, G, and B pixel devices operate at different LC layer thicknesses is usually adopted. FIG. 3 depicts a schematic cross-section view of such RGB pixel devices in accordance with the method targeting on varying LC layer thickness. As shown, each pixel device comprises an upper substrate 100, a lower substrate 300, and an interposed LC layer 200. The upper substrate 100 has a color filter layer 110 formed on a lower surface thereof to let the pixel device shows a preset color. The lower substrate 300 has a transparency organic layer 312 formed on an upper surface thereof. Pairing of a pixel electrode layer 310 formed on the transparency organic layer 312 and a common electrode layer 120 under the color filter layer 110 represents an electric field E to drive the LC layer 200. Furthermore, two alignment films 130 and 320 are formed respectively on the inner surfaces of the common electrode layer 120 and the pixel electrode layer 310 to set the orientation of the LC layer 200. As shown, the RGB pixel devices assign various thicknesses to the organic layers 312 so as to vary local thickness of the LC layer 200.
It is well understood that the spacing between the pixel electrode layer 310 and the common electrode layer 120 can severely affect the strength of the electric field E formed in the LC layer 200. Also, it is clear that the thickness of the LC layer 200 and the strength of the electric field E are both related to the transparency of the LC layer 200. Therefore, by assigning different thicknesses to the color filters 110 or the transparency organic layers 312 in the RGB pixel devices, the transparency of the LC layer 200 can then be adjusted and thereby the RGB gamma curves as shown in FIG. 1 can have better coherence.
However, the above-described gamma adjusting method has the following drawbacks.
1. For additional steps of forming transparency organic layers 312 with different thicknesses are demanded before the step of forming the pixel electrode layer 310, so the fabrication cost will definitely increase.
2. The transparency layers 312 with various thicknesses are provided with a rough upper surface on which the alignment film 320 is hard to form.
3. The pixel electrode layer 310 on the transparency organic layers 312 is formed on a rough surface. Therefore, a lateral electric field may exist between the neighboring pixel electrodes, and thus disturbs normal operation of the pixel devices.
4. Because the color filter layers 110 absorb part of the illumination passing through, the RGB pixel devices may present different displaying brightness due to the thickness variation of the color filter layers 110. Moreover, the color filter layers 110 are also provided with a rough exposed surface, which may make it difficult to form the alignment film 130 thereon.
The above-described gamma correction method utilizes structural change, particularly in the LC layer thickness, upon the pixel devices to overcome possible curve-separation phenomenon shown in FIG. 1. On the other hand, another method for adjusting gamma curves by compensating the level of digital displaying signals is also available in the art. The method proceeds by increasing or decreasing the levels of RGB digital displaying signals to some extent before they are converted into analog ones for driving the pixel devices. However, limited segmentations upon the levels may put some close but different inputted digital displaying signals to have an identical output. Therefore, some displaying levels may be sacrificed and also the color resolution of the LCD would be declined.
Other than the above-described method, another gamma correction method is introduced to utilize a technique of adjusting the reference electric levels applied when the digital displaying signals are converted into analog ones. As mentioned, the reference electric level builds up the relationship between the digital displaying signals and the analog ones, and thus it may influence the output analog signals. However, empirically, because the level of digital displaying signal versus the transparency of the LC layer presents a non-linearly relationship, a demand of dynamic adjustment circuits to calibrate the reference electric levels is highly expected.
In order to build up the so-called non-linearly relationship, there needs at least one adjustable reference electric level circuits respect to R, G, B displaying colors, and each of the reference electric level circuits should be able to adjust the corresponding reference electric level from the level of the lowest digital displaying signal to the highest displaying signal. Therefore, an enormous amount of adjustable levels are needed for each the reference electric level circuit and thus complicates the corresponding LC driving circuit.